The present invention relates to the measurement of video signals, and more particularly to a method of controlling and detecting alignment of composite video color frames when doing picture quality measurements using a differencing technique.
When measuring picture quality via a differencing technique, as in the PQA200 Picture Quality Analyzer manufactured by Tektronix, Inc. of Wilsonville, Oreg. and described in U.S. Pat. No. 5,18,520 issued Oct. 6, 1998 entitled "Programmable Instrument for Automatic Measurement of Compressed Video Quality", if the video system being measured has signals that are composite encoded per the NTSC, PAL or SECAM television standards, then the phase of the composite color frame sequence relative to a test reference sequence needs to be known and controlled. This problem is an extension of the general problem seen first in the analog component world where, if there are multiple generations of encode and decode between composite and component, then the phase of the original encoding must be preserved to insure quality video reproduction.
The root of the problem is that component signals are inherently only two-field sequences, whereas composite encoded NTSC is a four-field sequence, composite encoded PAL is an eight-field sequence and composite encoded SECAM is a twelve-field sequence as shown in FIG. 1. Many artifacts of the encode and decode process are different as a function of the subcarrier phase. Since the subcarrier phase changes over the color frame, if two encode and decode cycles are used with different color frame phase, then the artifacts interact and significantly reduce video quality. Thus the color frame phase of subsequent encode and decode cycles needs to be kept the same as the original.
This color frame phasing has historically been accomplished by using the color frame rate signal "Black Burst" as a reference for studio applications. This insures that all encoders use the same phase of the subcarrier burst when encoding. Decoders sense the color frame phase from the incoming composite signal. As the world moves to digital component video and MPEG compressed video, the black burst reference remains in studio use as the method to allow color frame phase control. The problem occurs in systems where the video is transmitted in a component format and there is no black burst reference on the receive end. In this case some other method is needed to allow this control. Examples of these other systems include MPEG satellite distribution to the home where there is no black burst reference available at the receiver, or any system where the encoders are not referenced to black burst.
Failure to control the color frame phase has two major problems in measuring video fidelity by a differencing technique. First, if the subcarrier phase used when storing a reference video sequence is different from that used when making a measurement of the video sequence from a system under test (SUT), then the encoder/decoder artifacts do not subtract out and the degradation measured is higher than it should be as it includes such artifacts. If the subcarrier phase is uncontrolled, then this results in, for example, bimodal distribution of results for NTSC and three or four modes for PAL.
The second problem is when there is a second set of encode and decode functions in the SUT. If the phase of the second encode does not match the phase of the first, then the interaction stated above occurs and serious video fidelity reduction results. The differencing technique correctly reports the reduction in video quality, but unless the cause is detected and reported customers may misinterpret or not trust the results.
What is desired is a color frame phasing control and detection method for aligning composite color frames when doing picture quality measurements using a differencing technique.